1. Field of the Invention
The present invention relates to satellite and terrestrial wireless modems and, more particularly, to a modem that can change its rates (hereinafter, a “rate adaptive modem”) using an adaptive technique, as subsequently explained and as referred to herein as “Information Throughput Adaptation” or (ITA).
2. Description of the Related Art
FIG. 1 illustrates a block diagram of satellite-based communication system using two conventional modems 4A, 4B, each located at a respective station A, B. Each station comprises a terminal 1A, 1B also having conventional amplifier, multiplexer and encoding capabilities, as well as an antenna 5A, 5B, respectively. The conventional modem 4A, 4B is a module coupled to other components of the terminal 1A, 1B, as an independent unit or integrated structure, between a source of user information and control data and an antenna 5A, 5B, respectively. The modem is adapted to modulate a carrier wave, which is generated at the terminal 1A, 1B and transmitted by the antenna, with the user information and control data, using well known techniques such as BPSK, QPSK, 8-PSK and 16-APSK. The modem also may encode the data according to techniques known to minimize losses and enhance accuracy, such as Viterbi and/or other codes at exemplary rates ½, ⅔, ¾, ⅞, 19/20. The modem also is adapted to demodulate a carrier wave received by the antenna, and strip away the data and information from the carrier wave, making the data and information available for further processing. In the illustrated conventional system, the user information and control data 2A that is input to user terminal 3A is first processed (encrypted, multiplexed, etc.) and then appended onto a carrier wave by modem 4A for transmission by antenna 5A to the satellite 6. The signal relayed by satellite 6 is received by antenna 5B, demodulated at modem 413 and processed (decrypted, demultiplexed, etc.) at user terminal 3B so that data and information 2B can be delivered for further processing. In bi-directional communications, modem 4B may also be used to modulate a carrier wave for transmission of data and information via the satellite 6, and modem 4A may be used to demodulate a received carrier wave for reception of those signals.
In a system using conventional modems, parameters such as symbol rate, type of waveform (modulation, coding) and data rates are fixed. Therefore, for communications to exist, both modems 4A and 4B must be initialized to the same set of configuration parameters. If the parameters need to be changed for any reason, the system must be taken off-line, i.e., the communications in the system between those two modems must be discontinued, before the new parameters can be entered.
Because the system must be taken off-line before parameters such as the user data rate can be changed, the conventional systems configure the modems with a modulation and coding such that the communication is reliable even in the worst of transmission conditions. Accordingly, the resultant user data rate is not maximized, i.e., bandwidth efficient, for all transmission conditions.
For example, FIG. 1 illustrates a storm 10 that is approaching the communications path that exists between antenna 5A and satellite 6. As illustrated in FIG. 2, the maximum possible user data rate for reliable communications and for a given set of atmospheric conditions is represented by curve 8. Curve 8 is at its maximum, point A, before the storm 10 approaches the communications path between antenna 5A and satellite 6 and after the storm 10 leaves the communications path between antenna 5A and satellite 6. As the storm 10 approaches the communications path, the user data rate must be lowered by changing the modulation, coding and symbol rate to less efficient values in order to maintain reliable communications. At the height of the storm 10, the user data rate must be lowered to the minimum point B in order to maintain reliable communications. In addition, a margin of error (or safety), which is illustrated as a difference between curves 7 and 8, is also employed so that the communications between the modems are as robust as possible. Therefore, to ensure reliable communications during the height of the storm 10, the user data rate must be lowered to point C.
Because the conventional modems do not allow for the user data rate to be changed dynamically, i.e., while communicating modems are on-line, and because taking the system off-line for every atmospheric change would be impractical, the user data rate in the conventional modems are set to the “worst case scenario,” i.e., line 9. Although this would ensure reliable communications, the modems are not bandwidth efficient for situations where the atmospheric conditions allow for higher user data rates.